Some Media keys

Untested

Laptop Settings

ACPI

Tp_smapi

Unfortunately, tp_smapi is only partially supported on the Thinkpad T420. A number of features work since version 0.41. For example, the hard drive protection mechanism HDAPS now works well. See the linked wiki entry.

Some features like setting the starting threshold for charging the battery do not yet work. To control the battery charging thresholds, install the Perl script tpacpi-batAUR from the AUR.

The example values 40 and 80 given here represent the percentage of full battery capacity remaining. Adjust them to your own needs. You may also want to write a simple set-battery.service and enable it to set them at startup. While these values should be permanent, they will be reset any time the battery is removed.

Laptop Mode Tools

Synaptics

TouchPad and TrackPoint do work out of the box, but the TouchPad is way too sensitive (i.e. fast) to be usable, since it is recognized as a mouse. To fix this, install the xf86-input-synaptics package and add the following two files to your /etc/X11/xorg.conf.d/ directory:

Turn touchpad on and off

For some, the (Fn+F8) key does not switch the touchpad on and off. Here is a simple keybind to add to your ~/.xbindkeysrc for keys to quickly change your touchpad state. For these to take effect, run xbindkeysrc. This binds Fn+F8 to 'toggle the touchpad on and off'. (Tested in i3wm and xfce4, where normal Fn+F8 does not toggle the touchpad)

Or try disable Laptop-mode.
Add !laptop-mode to the DAEMONS array in /etc/rc.conf:

DAEMONS=(...!laptop-mode...)

This forum post details another way to have your computer not reboot on shutdown. Turning off the laptop-mode daemon causes battery life to suffer, so when on the move and in need of a simple way to shutdown, this seems to work better.

Hang on reboot

This is a problem on many laptops and can be fixed by blacklisting the e1000e kernel module.

No backlight controls

One user has reported that the brightness controls (fn+home, fn+end) did not work in some desktop environments. This could be fixed by adding the following kernel options:

acpi_backlight=vendor acpi_osi=Linux

Also try to adjust the display in the console and not in X windows. If you don't have the keybindings correct in X, it will cause a problem. This can be bypassed if you use a virtual console to adjust the brightness.